This invention relates to an optical recording disk, especially to a phase-change erasable type system and also relates to a method and apparatus for manufacturing the same.
Erasable optical disk systems will embody all of the attributes of write-one and have the capability of direct file replacement of current magnetic systems. Full realization of the potential of optical disk systems awaits the availability of erasable systems.
Nowadays, two major families of erasable media are available: one is 1) phase-change erasable (PCE) system and the other 2) magnetooptical (MO) erasable system. Data recording on PCE media relies on the transition from a crystalline phase to an amorphous phase. On the other hand, MO recording utilizes the polarization of the laser beam magnetically modulated in media which display the Kerr effect. The principal characteristics of erasable disk are shown in the below Table.
TABLE ______________________________________ PCE MO ______________________________________ Recording and Phase change Change of erasing mode magnetization Read Change of Change of amplitude polarization Material of medium Te--Ge--Sb Tb--Fe--Ni--Co Operation mechanism Simple Complicated Magnetic field Not required Required Reversibility Fair Good Require laser power High Medium ______________________________________
In MO systems, there are four representative references as the prior art; Firstly, Yamada'320 shows that targets can be alloy formed of TbFeCo or one of the targets is formed Fe and the other target can be TbCo. The method is carried out by combination of Rf sputtering and DC sputtering, the former makes a resulting layer to have a high density and thus be strongly adhesive to a base metal while the latter makes the resulting layer to be formed at a higher velocity and a lower temperature. Secondly, Sato'755 shows that a magnetic recording material can be formed by simultaneous sputtering from Tb target and FeCo target. Thirdly, Japanese patent Tokkaihei 1-184742 shows that a magneto-optical material can be formed by simultaneous sputtering from different compositional TbFeCo alloy targets. Fourthly, Steininger'846 shows a method for preparing a prospective film useful to a magneto-optical recording material.
However, the material for MO systems is TbFeCo or TbFeNiCo different from PCE material, TeGeSb because the MO disk drive requires a complicated system for applying the writing magnetic field with a polarity opposite to the erasing magnetic field while the PCE disk drive requires a simple system for applying the laser with a modulated intensity between a recording one and an erasing one.
Therefore, with a PCE disk, the overwrite mechanism is easy to design and reversibility is fair compared to the MO disk, so that the present invention is to provide a PCE disk with the superior overwrite system.
The overwrite system in the PCE disk is as follows. The laser intensity is modulated between a recording intensity and an erasing intensity. When the intensity is a recording level, the material having a recording signal reaches a melting temperature and then is quenched rapidly to be in the amorphous phases, giving low surface reflectivity in which a new signal is directly overwritten on the old signal and recorded. On the other hand, when the intensity is an erasing level, the material is heated up to more than the crystallization point, allowing rapid crystallization to proceed, giving an increased surface reflectivity, so that the old signal is erased. As seen from this, the crystallization velocity of the disk material is a very important parameter for carrying out the overwrite method. If the crystallization velocity is too slow (i.e. the crystallization time is too long) compared with the irradiation time of the laser, the material can not be sufficiently crystallized to have no-erased part. Contrary to this, if the crystallization velocity is too fast (i.e. the crystallization time is too short) compared with the irradiation time of the laser, the material is recrystallized and can not be sufficiently changed to an amorphous phase even if heated to be melt, resulting in insufficient recording state or distorted recording style. Accordingly, the linear velocity (the relative velocity of the optical disk against the laser irradiation) should be chosen to correspond with the inherent crystallization velocity of the disk material.
As the material of the PCE disk there has been known GeSbTe ternary alloy, which thin film has a changeable crystallization velocity as shown in FIG. 3 reported in Japanese Journal of Applied Physics, Vol.26 (1987) Supplement 26-4, page 61. According to FIG. 3, the crystallization velocity can be changed along the line connecting between the GeTe point and the Sb2Te3 point thereof. Seen from FIG. 3, richer GeTe the alloy contains, slower the crystallization velocity becomes. On the other hand, richer Sb2Te3 the alloy contains, faster the crystallization velocity becomes.
Therefore, on the basis of the above theory, there has been proposed a method for controlling the crystallization velocity of the PCE disk in Japanese Patent Tokuganhei 1-184510, wherein the disk has a characteristic that the crystallization velocity is designed by means of adjusting the compositional ratio of GeTe and Sb2Te3.
However, after our further investigation there has been found problems that the above manner of changing the crystallization velocity in the radical direction of the disk makes amplitudes of the regenerative signal different with the crystallization velocity, which results in obtaining no homogeneous regenerative quality on all the recording area of the optical disk. The reason is that the compositional ratio between GeTe and Sb2Te3 changes not only the crystallization velocity of the recording film as shown above but also the complex index of refraction so much, which causes change of reflectance in the crystallization phase and the amorphous phase, resulting in change of amplitude of the regenerative signal, because the signal regeneration in the phase change optical disk system is carried out by detecting the difference of reflectance between the crystallization phase and the amorphous phase on irradiation of the laser.
A first object of the present invention is to provide a method for manufacturing the PCE optical under control of not only the crystallization velocity but also the complex index of refraction and thus the reflectance on the disk surface.
Meanwhile, there are two manners of rotating the optical disk for write/read; one is to keep the constant linear velocity at all the area of the disk during operation. The other is to keep the constant angular velocity (the constant rotational numbers) during operation. According to the latter method, a higher operation can be done because of no adjustment required even if the optical head moves from inside area to outside area of the disk in order to make access to necessary informations. Therefore, the latter method is recommendable. However, if the optical disk rotates at the constant angular velocity, the linear velocity changes between the inside area and the outside area. That is, it runs slower at the inside area and faster at the outside area. In such case, if the disk has the crystallization velocity corresponding to the inside linear velocity, non-erased part remains due to short of crystallization. On the other hand, if the disk has the crystallization velocity corresponding to the outside area, recrystallization of the melt part proceeds at the inside area and insufficient recording mark might be obtained.
To solve the problem, according to Japanese Patent Tokuganhei 1-184510, it is proposed that the recording film composition should be changed to adjust the crystallization velocity in the radial direction with richer GeTe content in the inside area and richer Sb2Te3 content in the outside area, resulting in good erasing characteristics from at all the area of the disk. As explained the above, however, the crystallization velocity in the radical direction of the disk makes amplitudes of the regenerative signal different with the crystallization velocity, which results in obtaining no homogeneous regenerative quality on all the recording area of the optical disk.
Therefore, a second object of the present invention is to provide a PCE optical disk suitable to be used under the constant angular velocity (the constant rotational numbers wherein the disk has a characteristic that the crystallization velocity is designed to become faster from inside area of the disk to outside area in the radical direction thereof under control of not only the crystallization velocity but also the complex index of refraction and thus the reflectance on the disk surface.
Further, according to the Japanese Patent Tokkaihei 1-184510, in order that the crystallization velocity is increased in a radial direction from inside area to outside area in the disk by means of sputtering from at least two targets at the same time, more than two evaporating or sputtering source are arranged at positions offset from the rotational center of the disk, so that higher rotation of the disk or slower formation rate of the disk must be adopted in order to make the disk to have a same composition in the circumferential direction of the disk. This is a big problem to be solved in a case of mass production and high film formation to be required.
Therefore, in the case of mass production and high film formation to be required, a third object of the present invention is to provide an effective method for manufacturing the optical disk by means of sputtering from at least two targets at the same time in a manner that the crystallization velocity is increased in a radial direction from inside area to outside area in the disk.